1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a moderator filled rod for a fuel assembly having a construction which enhances the effectiveness and reliability of the rod and which also extends its useful life.
2. Description of the Prior Art
In a typical nuclear reactor, such as the pressurized-water type (PWR), an excessive amount of reactivity is designed into the reactor core at start-up so that as the reactivity is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. Commonly, burnable poison or neutron absorber rods are used in the reactor core during the first cycle or beginning of life (BOL) of the reactor core in order to reduce excess reactivity. Poison material has high-absorption cross section that absorbs neutrons unproductively so as to reduce the reactivity of the reactor. After the excessive reactivity has depleted, then, during the second cycle of reactor operation, no devices are added to modify the level of reactivity. Once reactivity has decreased still further, then finally, in the third cycle or end of life (EOL) of the reactor operation, additional moderator is introduced to increase the reactivity of the core. Moderator material slows down neutrons from the high energies at which they were released. The neutrons lose energy by scattering collisions with nuclei of the moderator. Slow neutrons or ones whose energies have been reduced to the thermal energy range have a better probability than fast ones of causing nuclear reactions. Thus, reduction of the speed of fast neutrons serves to increase reactivity.
Therefore, in PWR fuel assemblies that have been irradiated to near EOL condition, e.g. third cycle fuel assemblies, it is desirable to exclude borated reactor coolant--which contains poison or neutron absorbing material--from the assemblies and to increase the amount of moderator in the fuel assemblies. One prior method of partially accomplishing this is to just remove several fuel rods from these assemblies at the beginning of the third cycle. Then, the empty space allows additional amounts of the moderator/coolant liquid mixture to be introduced into the core. However, while this increases the amount of moderator in the fuel assemblies, it does not reduce the amount of poison (borated coolant) in the assemblies; in fact, it increases the amount of poison since a proportional increase in the amount of borated coolant in the fuel assemblies accompanies the increase in the amount of the moderator.
Consequently, a need exits for a more effective way to introduce moderator into the reactor core at the beginning of its EOL or third cycle.